<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .reg {
            width: 400px;
            height: 200px;
            margin: 0 auto;
        }
        .reg input {
            outline: none;
            border:1px solid skyblue;
        }
        .reg span {
            font-size: 12px;
        }
        .reg span::before {
            content: '';
            display: inline-block;
            width: 16px;
            vertical-align: middle;
            height: 16px;
            background: url(images/mess.png) no-repeat;
        }
    </style>
</head>
<body>
    <div class="reg">
         <input type="text" placeholder="输入账号名">
         <span>请输入6~16位密码</span>
    </div>
</body>
</html>